The large scale growth of semiconducting thin films is the basis of modern electronics and optoelectronics. Reducing film thickness to the ultimate limit of the atomic, sub-nanometer length scale, a difficult limit for traditional semiconductors (e.g., Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technology. For this, transition metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers (MLs), provide semiconducting materials with high electrical carrier mobility, and their large-scale growth on insulating substrates would enable batch fabrication of atomically-thin high-performance transistors and photodetectors on a technologically relevant scale without film transfer. In addition, their unique electronic band structures provide novel ways to enhance the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of ML TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge.
Existing growth methods for large-scale ML TMDs have so far produced materials with limited spatial uniformity and electrical performance. For instance, the sulphurization of metal or metal compounds only provides control over the average layer number, producing spatially-inhomogeneous mixtures of mono-, multi-layer and no-growth regions. While chemical vapour deposition (CVD) based on solid-phase precursors (e.g. MoO3, MoCl5, or WO3) has demonstrated better thickness control over large scale, the electrical performance of the resulting material, which is often reported from a small number of devices in selected areas, fails to show spatially uniform high carrier mobility.